Polymerization of acrylonitrile, bulk

Bulk Polymerization. The bulk polymerization of acrylonitrile is complex. Even after many investigations into the kinetics of the polymerization, it is stiU not completely understood. The complexity arises because the polymer precipitates from the reaction mixture barely swollen by its monomer. The heterogeneity has led to kinetics that deviate from the normal and which can be interpreted in several ways. 

Although bulk polymerization of acrylonitrile seems adaptable, it is rarely used commercially because the autocatalytic nature of the reaction makes it difficult to control. This, combined with the fact that the rate of heat generated per unit volume is very high, makes large-scale commercial operations difficult to engineer. Lastiy, the viscosity of the medium becomes very high at conversion levels above 40 to 50%. Therefore commercial operation at low conversion requires an extensive monomer recovery operation. 

Similarly, Garcia-Rubio and Hamielec (17) conducted bulk polymerizations of acrylonitrile at various temperatures and initiator levels in glass ampoules. Their plots of the rate of polymerization as a function of conversion are typical of the extensive radical occlusion in this very glassy polymer. 

Poly(acrylic acid) is not soluble in its monomer and in the course of the bulk polymerization of acrylic acid the polymer separates as a fine powder. The conversion curves exhibit an initial auto-acceleration followed by a long pseudo-stationary process ( 3). This behaviour is very similar to that observed earlier in the bulk polymerization of acrylonitrile. The non-ideal kinetic relationships determined experimentally in the polymerization of these two monomers are summarized in Table I. It clearly appears that the kinetic features observed in both systems are strikingly similar. In addition, the poly(acrylic acid) formed in bulk over a fairly broad range of temperatures (20 to 76°C) exhibits a high degree of syndiotacticity and can be crystallized readily (3). 

The bulk polymerization of acrylonitrile has been studied by numerous workers (for a literature survey on the problem see ref. j O and JL1J. The kinetic features of this reaction at room temperature are summarized in Table I. It is one of the typical examples of polymerization under heterogeneous conditions in which the anomalies are generally assumed to arise as a result of non-stationary conditions caused by the "occlusion" of growing chains in the precipitated polymer (10). The presence of occluded radicals was indeed demonstrated by 5R measurements (12) and by... 

The cfata presented in Table I show that the kinetic features of the bulk polymerization of acrylonitrile are very similar to those observed with acrylic acid. It therefore seems pertinent to query whether atrix effect could not arise in the polymerization of acrylonitrile through a regular orientation of monomer molecules along the polymeric matrix involving dipole interaction of the -CsN groups (structure IV). 

The bulk polymerization of acrylonitrile in this range of temperatures exhibits kinetic features very similar to those observed with acrylic acid (cf. Table I). The very low over-all activation energies (11.3 and 12.5 Kj.mole-l) found in both systems suggest a high temperature coefficient for the termination step such as would be expected for a diffusion controlled bimolecular reaction involving two polymeric radicals. It follows that for these systems, in which radicals disappear rapidly and where the post-polymerization is strongly reduced, the concepts of nonsteady-state and of occluded polymer chains can hardly explain the observed auto-acceleration. Hence the auto-acceleration of acrylonitrile which persists above 60°C and exhibits the same "autoacceleration index" as at lower temperatures has to be accounted for by another cause. 

Bulk polymerization of acrylonitrile initiated by gamma radiation was described by Chapiro at The process is greatly complicated by other reactions. Occlusion of... 

Example Bulk polymerization of Bulk polymerization of acrylonitrile... 

Table I. Specific Surface of Particles Formed in Bulk Polymerization of Acrylonitrile at 50°C.

Figure 4. Bulk polymerization of acrylonitrile at 50°C. AIBN concentration, 2.07 X 10 sM...

Figure 5. Rate of bulk polymerization of acrylonitrile at 50°C. as a function of AIBN concentration...

While it is assumed that termination by coupling takes place when maleic anhydride and styrene are copolymerized in a good solvent such as acetone, insoluble macroradicals precipitate when these monomers are copolymerized in a poor solvent such as benzene (7). Insoluble macroradicals obtained by bulk polymerization of acrylonitrile (1, 11) and the solution copolymerization of maleic anhydride and styrene in benzene (7) have been used as seeds for the preparation of block copolymers. 

Recipes. The minimum number of chemically different components for heterophase pol5unerizations is two. This is the case if a monomer, which is a nonsolvent for its own polymer is polymerized in bulk. Light-induced bulk polymerization of acrylonitrile and vinyl chloride might be examples for this procedure. However, well-defined products especially with regard to colloidal properties, require more efforts. In order to carry out reproducible heterophase polymerization up to high conversions with highly controlled polymeric and colloidal properties, at... 

Various inhibitors have been used in new determinations of rates of initiation in bulk polymerizations of acrylonitrile " and solution polymerizations of styrene an improved procedure has been proposed.Aromatic aldonitrones, polymers with attached t-butyl nitroxide groups and charge-transfer complexes of anthracene have been studied as inhibitors. 

Pistoia has used electrochemically generated nitrate radicals to effect the bulk polymerization of acrylonitrile the system shows a remarkable postpolymerization effect which is affected by such factors as the anode material, current, temperature, stirring, electrolysis time, and HNO, concentration. Radical occlusion phenoma and the formation of oligomers limit the monomer to >oly-mer conversion. Pistoia has also reported the polymerization of acrylonitrile by the oxidation of sulphuric acid at the anode and has extended the work to the anodic polymerization of methyl methacrylate in methanol-sulphuric acid... 

Like poly(vinyl chloride), polyacrylonitrile does not dissolve in its own monomer hence, when bulk polymerized, the polymer precipitates from the monomer solution. The precipitated polymer particles, if not stabilized, tend to agglomerate to form a polymer paste or slurry. There are two polymerization reaction loci a polymer-free monomer phase and a polymer phase containing dissolved monomer. The polymer phase may be saturated with monomer. The physical properties of the polymer and monomer phase have a significant effect on the polymerization rate. The polymer particles formed in bulk polymerization of acrylonitrile consist of polymeric phase made up of 94% polyacrylonitrile and 6% acrylonitrile [54]. Due to its extremely low solubility in acrylonitrile, the polyacrylonitrile polymer is believed to grow on the polymer particle surface from the very low conversion. If the coagulation of initial small precipitated particles occurs, the reaction site will be either the outer or inner surface of a coagulated particle. 

Figure 13 Polymerization rate curves for bulk polymerization of acrylonitrile (60°C with AIBN initiator). (From Ref. 56.)...

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